The present invention relates to a method of converting transverse electric modes and to a helically cut aperture antenna for implementing the method. More particularly, the present invention relates to a method of converting transverse electrical modes which are then radiated by the aperture antenna into the space in front of it.
Electron cyclotron resonance heating elements for plasma fusion experiments require a high power of several megawatts at a frequency of about 140 GHz; this power is generated by gyrotrons. The typical gyrotron operating modes are transverse electric modes TE.sub.mn having a high first (azimuthal) index (m) and a relatively low (radial) index (n). Such modes are not suitable for heating plasma, but rather must be converted into a linearly polarized, approximately Gaussian beam.
An article by M. Thumm, "Computer-Aided Analysis and Design of Corrugated TE.sub.11 to HE.sub.11 Mode Converters in Highly Overmoded Waveguides", International Journal of Infrared and Millimeter Waves, Volume 6 (1985) pages 577-597, recommends a method with which transverse electric modes can be converted into hybrid modes by a mode converter comprising a circular waveguide with circumferential or annular corrugations on its interior surface whose depth gradually decreases from one half to one quarter of the wavelength in the direction of propagation toward an aperture antenna.
So-called Vlasov converters have been used in the past for a quasi optical conversion. Such a converter is composed of a helically cut aperture antenna connected to a waveguide end, and one or a plurality of reflectors in the beam path (see: S. N. Vlasov et al, "Transformation of a Whispering Gallery Mode, Propagating in a Circular Waveguide, into a Beam of Waves", Radio Engineering, Electron Physics, Vol. 21, 1975, pages 14-17).
In the intended emission of rotating transverse electrical modes, undesirable sidelobes of the beam characteristic appear in the far field (Fraunhofer region). Moreover, at the high emission energy, the antenna is subjected to stresses which cannot be managed without direct cooling measures.